1. Field of the Invention
The present invention relates to an image forming apparatus which collects residual toners on a latent image carrier by a developing unit.
2. Description of the Related Art
In image recording apparatuses, such as a copying machine, a printer and a facsimile, a latent image forming apparatus like an electrophotographic apparatus is used due to the popularity of image recording on normal sheets of paper. Such an image forming apparatus forms an electrostatic latent image on a photosensitive drum or the like. Then, the electrostatic latent image on the photosensitive drum is developed with a powder developer to provide a visible image. The developed image on the photosensitive drum is then transferred on a sheet, and the sheet is then separated from the photosensitive drum. Thereafter, the developed image on the sheet is fixed.
Due to the recent demand for compact image forming apparatuses and economical usage of resources, it is desirable to employ a method that does not dispose of residual toners on a photosensitive drum.
FIGS. 27A and 27B are explanatory diagrams of prior arts, and FIG. 28 is an explanatory diagram of a cleanerless process.
The conventional image forming apparatuses will be described below as latent image forming apparatuses with reference to a typical electrophotographic printer. As shown in FIG. 27A, various types of process units are disposed around a photosensitive drum 1, such as an organic photosensitive body, Se photosensitive body or a-Si photosensitive body. More specifically, arranged around the photosensitive drum 1 are a precharger 2, an image exposing unit 3, a developing unit 4, a transfer unit 5, a cleaner 6 and a de-electrifying lamp 7.
In the image forming operation, the photosensitive drum 1 is charged by precharger 2, and is exposed by the exposing unit 3 so that an electrostatic latent image is formed on the drum 1 according to the exposed image. The developing unit 4 supplies a powdery developer (e.g., one-component magnetic toners or a two-component developer) to the photosensitive drum 1 to develop the electrostatic latent image. The transfer unit 5 transfers the developed image on the photosensitive drum 1 onto a sheet P that is fed in the arrowhead direction. The sheet carrying the transferred image is then fed to a fixing unit 8 where the developed image is fixed on the sheet.
The efficiency of transferring the toner image on a sheet of paper is not 100%, and some toners will remain on the photosensitive drum. Therefore, the top surface of the photosensitive drum 1 after the toner image transfer on the sheet is cleaned with the cleaner 6 to remove the residual toners. Then, the de-electrifying lamp 7 is activated to remove the residual charges on the photosensitive drum 1 to return the drum 1 to the initialized state to be ready for another printing operation.
The residual toners collected from the photosensitive drum 1 by the cleaner 6 are temporarily stored in a waste-toner tank by a toner carrying mechanism (not shown), and a user will dispose of this tank when a predetermined amount of waste toners is stored.
This image forming process requires a toner disposal mechanism and space for storing the waste toners, and stands in the way of making the image forming apparatus compact. As the toners collected by the cleaner 6 do not contribute to printing, this process is not economical. Further, the disposal of the toners will raise an environmental problem.
In view of the above and due to the recent demands for smaller apparatuses and lower cost, it is desirable to eliminate part of the recording process. As one solution, a cleanerless process to eliminate the need for the cleaner has been proposed as in, for example, "Cleanerless Laser Printer," Electrophotographic Institute Report, vol. 30, no. 3, pp. 293-301.
This cleanerless process eliminates the use of the cleaner 6, and allows the residual toners after image transfer to be collected by the developing unit 4 so that the residual toners can be used again for printing. As shown in FIG. 27B, the cleaner 6 is eliminated and a conductive uniform distribution brush 9 is provided instead in the cleanerless process.
In this recording process, the residual toners on the photosensitive drum 1 are distributed by the distribution brush 9. Then, the surface of the photosensitive drum 1 with toners thereon is uniformly charged by the corona charger 2, an image exposure is performed by the exposing unit 3, and the collection of the residual toners after image transfer and the developing of the electrostatic latent image are carried out at the same time by the developing unit 4.
The toners concentrating locally on the photosensitive drum 1 are distributed by the distribution brush 9 to reduce the amount of toners per unit area, thereby facilitating the toner collection by the developing unit 4. Further, as the toners are distributed, the residual toners are prevented from becoming a filter for ion shower from the corona charger 2 to thereby avoid non-uniform charging. Also, the toners in the exposing step are prevented from becoming a filter to thereby avoid uneven exposure.
The point of this recording process is the collection of the residual toners on the photosensitive drum 1 performed at the same time as the developing step. This point will be described with reference to FIG. 28 which shows the photosensitive drum 1 charged negatively with toners also charged negatively. The surface potential of the photosensitive drum 1 is set to -500 to -1000 V by the charger 2, and the potential of the exposed portion where the potential drop has occurred due to the image exposure is dropped down to 0 to minus several tens of volts, thereby forming an electrostatic latent image. At the developing time, a developing bias voltage (e.g., -300 V) lying nearly in the middle of the surface potential and the latent image potential is applied to the developing rollers of the developing unit 4.
In the developing step, the negatively-charged toners on the developing roller stick on the electrostatic latent image on the photosensitive drum 1 by an electric field formed by the developing bias potential and latent image potential, thereby providing a toner image.
In the cleanerless process, at the same time as the developing step is performed, the residual toners after image transfer, distributed over the photosensitive drum 1 by the distribution brush 9 in the uniform distribution process, are collected to the developing roller from the surface of the drum 1 by the electric field that is created by the surface potential and the developing bias potential.
In the above cleanerless process, (1) no mechanism for disposing toners is required, thus contributing to making the printer compact, (2) no space required to store the disposed toners, (3) all the toners will be used in printing, which is economical, (4) no toners will be disposed of, which is friendly to the environmental preservation, (5) no cleaner, which scrapes the surface of the photosensitive drum 20 to shorten the service life thereof, is used to thereby elongate the life of the drum 20.
According to the prior arts, it has been proposed to employ a one-component non-magnetic developing scheme and a two-component developing scheme which uses non-magnetic toners and magnetic carriers, in this cleanerless process.
According to the conventional one-component non-magnetic developing scheme, the collecting power in the developing process is the electrostatic force which is created by the charged potential of the non-magnetic toners and the developing bias potential. This collecting power is therefore weak and the amount of uncollected toners increases, so that an afterimage is formed.
According to the conventional two-component developing scheme which uses non-magnetic, the collecting power in the developing process is the electrostatic force plus the scraping force by the magnetic brush. Likewise, this collecting power is weak and the amount of uncollected toners increases, so that an afterimage is formed.
In either developing method, toners charged to the opposite potential and low-charged toners, which are apt to stick on the photosensitive drum, and those types of toners cannot be collected by the electrostatic force alone.